According to Applied Optics 26 (1987), pages 845-849, a method is known for coating semiconductor lasers with anti-reflection layers, specifically semiconductor lasers to be used in a system for coherent optical transmission. For this, the surface reflection at the light emitting sides must be minimized, especially if the semiconductor laser is the part of an external resonator. The refraction index of the layer must be selected with reference to the wavelength of the laser light emitted by the semiconductor laser and the refraction index of the wave conducting layer of the semiconductor laser.
A semiconductor laser, positioned in a reaction chamber of a coating reactor, is coated with silicon nitride. During the coating an alternating current is applied to the semiconductor laser, and the light emitted thereby by the semiconductor laser, filtered by an interference filter, is measured by a photodiode located outside the reaction chamber. The electric signal generated by the photodiode is made symmetric in a converter and is fed into a lock-in amplifier. The reflection capabilities and the thicknesses of the layers can be determined from the amplifier output signal.
The disadvantage of this method is in the high experimental expenditure required for the determination of the layer thicknesses, which is obtained through conversion and re-conversion between electrical and optical signals.
[German patent publication] DE 38 02 841 A1 describes a method for interpretation of electrical noise spectra, by which the optical properties of semiconductor lasers can be determined. However, this method is applicable only to semiconductor lasers already manufactured. There is no indication referring to manufacturing steps for semiconductor lasers based on interpretation of noise spectra.